[2] [2] PENG M,SUN Y,LI X,et al. Recent advances in cloud radio access networks:system architectures,key techniques,and open issues[J]. IEEE Communications Surveys Tutorials, 2016,18(3):2282-2308.

[3] [3] HU B,HUA C,ZHANG J,et al. Joint fronthaul multicast beamforming and user-centric clustering in downlink C-RANs[J]. IEEE Transactions on Wireless Communications, 2017,16(8):5395-5409.

[5] [5] QUEK T,PENG M,SIMEONE O,et al. Cloud radio access networks:principles,technologies,and applications[M]. Cambridge, USA:Cambridge University Press, 2017.

[6] [6] LIU D,WANG L,CHEN Y,et al. User association in 5G networks:a survey and an outlook[J]. IEEE Communications Surveys and Tutorials, 2018,18(2):1018-1044.

[8] [8] DAI B,YU W. Energy ef?ciency of downlink transmission strategies for cloud radio access networks[J]. Journal on Selected Areas Communications, 2016,34(4):1037-1050.

[9] [9] ZUO J,ZHANG J,YUEN C. Energy ef?cient user association for cloud radio access networks[J]. IEEE Access, 2016,4(8): 2429-2438.

[10] [10] LIU L,YU W. Cross-layer design for downlink multihop cloud radio access networks with network coding[J]. IEEE Transactions on Signal Processing, 2017,65(7):1728-1740.

[12] [12] PARK S H,SIMEONE O,SAHIN O,et al. Joint precoding and multivariate backhaul compression for the downlink of cloud radio access networks[J]. IEEE Transactions on Signal Processing, 2013,61(22):5646-5658.

[13] [13] AHLSWEDE R,CAI N,LI S Y R,et al. Network information flow[J]. IEEE Transactions on Information Theory, 2015,46(4):1204-1216.

[14] [14] YAN Z,PENG M,WANG C. Economical energy ef?ciency: an advanced performance metric for 5G systems[J]. IEEE Wireless Communications, 2017,24(1): 32-37.